Add LINE Messaging API Alerter

[nsano-rururu]

Description

Support for LINE Messaging API, an alternative service to Line Notify, has been added.

Checklist

• I have reviewed the contributing guidelines.
• I have included unit tests for my changes or additions.
• I have successfully run make test-docker with my changes.
• I have manually tested all relevant modes of the change in this PR.
• I have updated the documentation.
• I have updated the changelog.

Questions or Comments

[Copilot]

Pull Request Overview

This pull request adds LINE Messaging API integration to ElastAlert2, enabling alerts to be sent to LINE applications via the LINE bot push message API.

• Implements a new LineMessageAPIAlerter class following the existing alerter patterns
• Adds comprehensive unit tests covering success cases, error handling, and required field validation
• Updates schema, documentation, and loader configuration to support the new alerter

Reviewed Changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 8 comments.

Show a summary per file

File	Description
elastalert/alerters/line.py	Implements the LINE Messaging API alerter with support for text message push notifications
tests/alerters/line_test.py	Provides unit tests for the LINE alerter including success cases, exception handling, field validation, and multiple match scenarios
elastalert/schema.yaml	Adds schema definitions for line_channel_access_token and line_to configuration options
elastalert/loaders.py	Registers the LINE alerter in the alerter registry and adds the module import
docs/source/alerts.rst	Documents the LINE Messaging API alerter configuration and usage examples

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[nsano-rururu]

@copilot open a new pull request to apply changes based on the comments in this thread

[jertel]

Thanks for adding LINE support back to the app.

This relates to #1638 and #1565.